Re: [PATCH v3 01/10] KVM: MMU: fix decoding cache type from MTRR

Paolo Bonzini
2015-07-13T07:32:13Z

On 12/07/2015 20:59, Xiao Guangrong wrote:
>
>
> On 07/13/2015 01:33 AM, Alex Williamson wrote:
>> On Wed, 2015-05-13 at 14:42 +0800, Xiao Guangrong wrote:
>>> There are some bugs in current get_mtrr_type();
>>> 1: bit 1 of mtrr_state->enabled is corresponding bit 11 of
>>> IA32_MTRR_DEF_TYPE MSR which completely control MTRR's enablement
>>> that means other bits are ignored if it is cleared
>>>
>>> 2: the fixed MTRR ranges are controlled by bit 0 of
>>> mtrr_state->enabled (bit 10 of IA32_MTRR_DEF_TYPE)
>>>
>>> 3: if MTRR is disabled, UC is applied to all of physical memory rather
>>> than mtrr_state->def_type
>>>
>>> Signed-off-by: Xiao Guangrong <guangrong.xiao@linux.intel.com>
>>> ---
>>> arch/x86/kvm/mmu.c | 14 ++++++--------
>>> 1 file changed, 6 insertions(+), 8 deletions(-)
>>
>>
>> I'm seeing a significant regression in boot performance on Intel
>> hardware with assigned devices that bisects back to this patch. There's
>> a long delay with Seabios between the version splash and execution of
>> option ROMs, and a _very_ long delay with OVMF before the display is
>> initialized. The delay is long enough that users are reporting their
>> previously working VM is hung with 100% CPU usage on v4.2-rc1. Thanks,
>>
>
> Alex, thanks for your report. I will try to reproduce and fix it asap.

The code that Bandan pointed out

+ /* MTRR is completely disabled, use UC for all of physical memory. */
+ if (!(mtrr_state->enabled & 0x2))
+ return MTRR_TYPE_UNCACHABLE;

actually disappears in commit fa61213746a7 (KVM: MTRR: simplify
kvm_mtrr_get_guest_memory_type, 2015-06-15). Should mtrr_default_type
actually be something like this:

static u8 mtrr_default_type(struct kvm_mtrr *mtrr_state)
{
if (mtrr_is_enabled(mtrr_state))
return mtrr_state->deftype & IA32_MTRR_DEF_TYPE_TYPE_MASK;
else
return MTRR_TYPE_UNCACHABLE;
}

? Then it's easy to add a quirk that makes the default WRITEBACK until
MTRRs are enabled.

Paolo